Graphite and other allotropes of carbon are the most widely used types of material used in the electrodes of batteries. However, in recent years, attention has turned to the development of electrodes from materials such as silicon, silicon-carbon composites, and lithium. Alkali metals, in particular lithium, are known to have the highest specific energy of all known electrode materials, making them a promising choice of material for use in the electrodes of batteries.
When metallic lithium is employed in the negative electrode of a battery, it is possible to use a wide variety of active materials in the positive counter-electrode. Such materials include non-lithiated manganese dioxide, vanadium dioxide, in addition to elements such sulphur, phosphorus, selenium, and so forth.
Furthermore, the use of metallic lithium as a negative electrode can significantly increase the specific energy of a battery. For example in a battery with lithium phosphate as the positive electrode, substituting a graphite negative electrode with a metallic lithium electrode can result in a 30% increase in the specific energy of the battery.
There are a number of drawbacks associated with the use of metallic lithium in electrodes. Metallic lithium is an extremely soft, plastic material that can easily be moulded into shape by pressing, extrusion and calendaring. However, handling metallic lithium during the manufacture of electrodes is a challenging task, since it adheres strongly to many of the structural materials involved. It can also be difficult to prevent bending and tearing, due to the low mechanical strength and extreme softness of metallic lithium.
Once assembled into an electrochemical cell, metallic lithium is prone to dendrite formation during battery charge. Dendrites are finely dispersed microscopic fibres or parts of lithium that are formed near the surface of the electrode. They are essentially excluded from involvement in the electrochemical processes occurring in the cell, and therefore reduce the specific energy of the battery. In some cases the dendrites can even reach the opposite electrode, resulting in potentially dangerous short-circuiting of the battery.
A further problem associated with dendrite formation is excessive reduction of the electrolyte by lithium. This results in passivation of the metallic lithium electrode and a need to increase the amount of electrolyte, which therefore reduces the specific energy of the battery.
There is an ever-present demand for increasingly smaller batteries that can supply power for longer periods of time, without the need for recharging. In particular, such batteries are highly desirable in portable electronic devices, for example smartphones and tablets, as they allow for more flexible and compact designs, whilst improving the performance of the device. In order to meet this need, it is necessary to find ways to increase the specific energy of batteries.
The specific energy of a battery or cell can be increased by reducing the overall mass of metallic lithium present in the electrode as much as possible, whilst maintaining the electrical capacity of the battery. This is typically achieved through the use of lithium foils, which can be calendared to reduce their thickness. However, due to the poor mechanical properties of metallic lithium, such technologies are only well developed for the preparation of lithium foils down to a thickness of 100 μm.
WO 2013/121164 describes a method for improving the mechanical properties of metallic lithium by incorporating polymer meshes as a reinforcement material. A sheet of polypropylene is placed between two sheets of lithium foil and the layers are pressure bonded together by calendaring. This method allows for the preparation of an electrode with a thickness of about 60 μm.
However, the manufacture of such electrodes using this method still presents a number of technical difficulties, for example such as handling lithium foils. Furthermore, the electrochemical properties of such electrodes leave room for improvement.
It is an object of the invention to address at least one of the above problems, or another problem associated with the prior art.